The cognitive and physiological mechanisms involved in the perception and recognition of visual stimuli will be studied. Ordinarily, people inspect the word with a series of short (200-300 msec.) fixation pauses of the eyes. These fixations alternate with the rapid saccadic eye movements that shift the gaze from one point to another. Normally, when a subject fixates on a stimulus, the image of the stimulus is present on the subject's retina for the total duration of the fixation. It is possible to interfere with this normal relation experimentally, by delaying the onset of a visual stimulus for a varying period after the beginning of a fixation, using a minicomputer-controlled cathode-ray display. The principal investigator has recently studied monkeys' visual behavior using this technique. The proposed experiments will extend this work to human subjects, by examining the effect on visual perception and visual fixation patterns of manipulating the stimulus within a single fixation. Stimulus onset delay and stimulus duration will be varied. By varying stimulus onset delay and duration of stimuli within a fixation, the effectiveness of visual information at different times within a fixation will be evaluated. Second, the role of visual information in controlling and initiating saccadic movements will be examined, and finally, the interaction of stimulus factors and task requirements with the other experimental questions will be studied.